Cross Laminated Timber

David
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Cross Laminated Timber, CLT for short, has become a competitive alternative to reinforced concrete panels in small to medium sized construction projects. Why is it competing with reinforced concrete? Quite simply timber when manufactured in the right way is very strong, relatively lightweight, has considerable environmental advantages and can be cheaper. Let’s take a closer look at the product and its characteristics.

Cross Laminated Timber, CLT for short, has become a competitive alternative to reinforced concrete panels in small to medium sized construction projects. Why is it competing with reinforced concrete? Quite simply timber when manufactured in the right way is very strong, relatively lightweight, has considerable environmental advantages and can be cheaper. Let’s take a closer look at the product and its characteristics.

The basic raw material will typically be a conifer softwood species such as Sitka Spruce, pine, fir, larch etc. Logs are processed in a sawmill and cut into rectangular elements with a particular width and thickness. The timber is dried to a specified moisture content before being planed to very precise widths and thicknesses. The ends of these elements are profiled with a zig-zag form. Glue is applied to the ends and then individual elements are forced together under great pressure. This results in a continuous length of glued timber. The continuous length of glued timber is then cut to specific lengths.

 

Cross Laminated Timber - Finger Jointed Timber

Cross Laminated Timber – Finger Jointed Timber

Next the finger jointed timber pieces are laid out in a layer, glue is applied along their side lengths and the pieces are forced together to ensure a very strong adhesion.

Cross Laminated Timber - Layer 1

Cross Laminated Timber – Layer 1

 

The next stage is to place another glued layer on top of the previous layer. But, the second layer will be oriented at 90° to the original. Glue is applied to the mating faces. This process is repeated until a panel of the specified thickness is generated. Pressure is applied to the panel to ensure excellent adhesion.

 

Cross Laminated Timber - Panel

The glue laminated panel is exceptionally strong. The fibres of the raw timber are aligned in layers and alternatively at 90° in every other layer. Any built in stress within the individual timber elements is cancelled out by adjacent timber elements. The final panel is dimensionally very stable and stress free. It is essential the tight quality and process controls are used throughout the manufacturing phase. This will ensure that the panel will not become prone to swelling due to the ingress of moisture. Timber panels have an intrinsic resistance to fire due to their bulk and the charring effect of timber. The charred outer layer, in the event of a fire, acts as an insulator.

Cross Laminated Timber - Fire performance

Cross Laminated Timber – Fire performance

 

Environmentally Cross laminated timber has one significant advantage over reinforced concrete. Namely that the raw material can be sourced from entirely sustainable and managed forest resources. FSC and PEFC approved forests are more or less carbon neutral. As trees are harvested they are replaced by newly planted trees which subsequently absorb the CO2 that will be released during the manufacturing and construction phases of a new project. Such CO2 releases arise from the power generation source – electricity.

The advent of powerful finite element analysis software that can model in great detail the way a cross laminated timber panel behaves under load has led to audacious proposed projects such as the Skidmore, Owings and Merrill (SOM) Timber Tower Research Project. This was a design for a 42 storey 395 ft. tall apartment building. The project demonstrated the feasibility of designing and building such a tall building utilising timber as the primary construction material. The building design utilises concrete floor-wall joints. The solid timber load bearing elements are connected with rebar via the concrete joints. The building is approximately 70% timber and 30% concrete by volume. The table below shows that the volume of structural material required for the cross laminated timber based building is efficient with respect to a benchmark concrete based building of the same dimensions.

Cross Laminated timber - Benchmark Comparison Table

Cross Laminated timber – Benchmark Comparison Table

 

The graph below provides a more visual comparison.

 

Cross Laminated Timber - Benchmark Graph

Cross Laminated Timber – Benchmark Graph

The drawing below shows the detail for the structural design.

 

Cross Laminated Timber - SOM Project Details

Cross Laminated Timber – SOM Project Details

The next drawing shows the how the floors of the structure are constructed.

 

Cross Laminated Timber SOM Project Floor Details

Cross Laminated Timber SOM Project Floor Details

The primary producers of cross laminated timber are based in Mainland Europe – Austria, Switzerland and Germany. North America is starting to produce the product. So availability will increase. It will be important that construction personnel become familiar and skilled with working with such a highly engineered product for the market share to increase.

National standards for the construction of buildings with cross laminated timber are being developed and will require very careful translation between countries to ensure that the detail is not lost. An outstanding resource is the CLT Handbook available for free download.

Constructive Translations helps clients by very accurately translating complex construction and engineering industry standards and technical documents.